1. Field of the Invention
The present invention relates to a method for measuring surface potential of a photosensitive body of an electrophotographic image forming apparatus and a method for effecting control on the basis of a measured result.
2. Related Background Art
Present electrophotographic image forming apparatus have widely been used not only as copying machines but also as printers as output means of computers or word processors which have recently been requested remarkably. There are various use conditions. For example, various image stabilizing means such as a means for preventing fluctuation of density of an output image caused by change in an environmental condition have been adopted. Particularly, in such printers, since not only normal office use but also personal use have been increased, low cost and maintenance free have been requested greatly.
Image quality of the output image from the electrophotographic system greatly depends upon sharpness or clearness in various electrophotographic processes, i.e., clearness of writing information, clearness of an electrostatic latent image formed on a surface of a photosensitive body, clearness of a color fine particle (referred to as "toner" hereinafter) image formed on the surface of the photosensitive body, clearness of the toner image after transferred to a transfer sheet and clearness of the image after the toner is fixed. Thus, in order to obtain a higher quality image, the clearness of various processes must be enhanced.
FIG. 72 is a schematic view of a known electrophotographic apparatus. In order to control an electrostatic latent image, methods for controlling a charging process or an exposure process on the basis of a result obtained by measuring potential of a surface of a photosensitive body within a macroscopic range.
A structure of the electrophotographic apparatus will now be described. The reference numeral 601 denotes a potential sensor; 602 denotes a support; and 603 denotes a photosensitive drum. The photosensitive drum 603 includes a conductive substrate 604, and a surface on which potential is varied. A surface 605 of the photosensitive drum 603 is charged by a charger 606, and an electrostatic latent image is formed on the drum by illuminating light from an image signal applying means 607 onto the drum. Distribution of the potential on the surface 605 of the photosensitive drum 603 is measured by the potential sensor 601. The electrostatic latent image is developed by a developing device 608 to form a toner image which is in turn transferred, by a transfer charger 610a, onto a transfer sheet conveyed from a transfer sheet supplying system 609 through a pair of resist rollers 614. After the transferring, the transfer sheet is separated from the drum by a separation charger 610b, and the separated transfer sheet is sent, through a transfer sheet conveying system 611, to a fixing device 615, where the toner image is fixed to the transfer sheet. Thereafter, the transfer sheet is discharged out of the electrophotographic apparatus. On the other hand, after the transferring, residual toner remaining on the surface 605 of the photosensitive drum 603 is scraped by a cleaning blade 616 of a cleaning unit 612. The remaining electrostatic latent image is erased by an electricity removing light source 613. In this way, a cycle of the processes is completed.
Now, the measurement of the potential of the surface of the photosensitive body using the potential sensor in the above-mentioned electrophotographic apparatus will be explained. First of all, in a condition that the light is not illuminated, the charging process is controlled to set the potential Vdark of the surface of the photosensitive body in a dark condition to a predetermined value. In this way, a reference charging condition at this point, i.e., a value of current or voltage to be applied is determined. Then, the photosensitive body is illuminated with a single light amount or plural light amounts, so that surface potential values of the photosensitive body are measured in the respective conditions.
From a relation between the exposure light amounts and the surface potential values of the photosensitive body so obtained, the reference light amount at this point is selected so that the surface potential V1 of the photosensitive body in the exposure becomes the predetermined value. Such potential measurement and potential control are performed upon rise-up of the electrophotographic apparatus and on demand. With this arrangement, the potential values Vdark and V1 are kept constant to prevent fluctuation of image density, thereby providing a stable image.
On the other hand, also in the printers, since output of not only characters but also photograph has been increased, higher image quality such as reproductively of fine dots or thin lines, as well as stability of electrophotography have been required more and more.
In order to achieve high image quality, it is required to control the electrostatic latent image on a small area of the surface of the photosensitive body. To do so, it is very important to know the fine shape of the electrostatic latent image.
However, since the electrostatic latent image cannot be observed directly and the dark portion of the latent image is maintained in the same condition only for a very short time, an observing means for judging whether the clearness of the electrostatic latent image formed on the surface of the photosensitive body is considerably limited. Thus, in the prior art, the following methods were utilized:
(1) method for visualizing the latent image in some manner; or
(2) method for making the electrostatic latent image quantitative during the electrophotographic process by using a potential measuring means.
As the above method (1), for example, a method in which the latent image is developed by using toner as the visualizing means and the image is evaluated to guess the electrostatic latent image before development has widely been used. However, in this method, a mechanism for measuring and analyzing the image after the development is required. Further, this method is not proper from the view point of increase in cost of the apparatus and poor reproductively, since dispersion in evaluation becomes great when the image is evaluated by the operator of the electrophotographic apparatus. In addition, this method is not preferably from the view point of environment, since a transfer material (copy sheet) and toner are required for evaluation.
As the above method (2), several methods for electrically evaluating the surface potential of the photosensitive body have been reported. For example, Japanese Patent Application (Laid Open) No. 5-508708 discloses a method for measuring evenness of the surface potential by using a detecting electrode. In this method, a potential sensor is arranged in the vicinity of a surface to be measured and dielectric current due to change in potential on the surface to be measured is generated on the potential sensor by effecting a relative movement between the sensor and the surface to be measured, and the evenness of the surface potential is measured by analyzing the result. The potential sensor has an edge, and it is characterized that the change in potential of the surface to be measured is detected by the edge of the detecting electrode.
Further, another example is disclosed in an electrophotographic institution magazine, volume 30, No. 2and No. 4 (1991). In this example, a means for preventing discharge by applying bias or for preventing discharge by suing gas (SF.sub.6) having high discharge start pressure is used and a potential sensor is arranged closely spaced apart form a surface to be measured by a distance of about 30 .mu.m and dielectric current due to change in potential on the surface to be measured is generated on the potential sensor by effecting a relative movement between the sensor and the surface to be measured, and change in gradient of potential of a latent image at an edge portion is measured by analyzing the result. It is characterized that the detection is effected by suing the potential sensor of several tens of .mu.m or less and the measurement gap of several tens of .mu.m or less.
In present electrophotography, particularly, digital electrophotography, the main resolving power is generally 600 dpi to 1200 dpi. It is required that the latent image potential so generated (referred to as "digital latent image potential" hereinafter), particularly, high image quality such as reproductively of fine dots and/or thin lines is maintained very stably against fluctuation of environment, thereby providing a sharp image stably.
As is in the above-mentioned conventional examples, in order to obtain the high quality image, it is required that configuration of the digital latent image potential formed by laser exposure having spot diameter smaller than several tens of .mu.m or an LED array, particularly, gradient of change in potential at a boundary between the non-exposed portion and the exposed portion of the digital electrostatic latent image (referred to as "sharpness of latent image" hereinafter), and, the potential difference between the non-exposed portion and the exposed portion, i.e., electrical depth of the latent image (referred to as "depth of latent image" hereinafter) are controlled and the image forming means is controlled in accordance with the depth and sharpness of the latent image.
To this end, similarly, a method for detecting distribution of potential of the latent image requires to detect the distribution of the potential of the digital latent image within a small range with a high resolving power, with the result that the control arrangement becomes complicated.
Further, for example, regarding a phenomenon "(high humidity) image flow" in which the image tends to become dim when the apparatus is sued under a high humidity environment, countermeasure is effected by frictionally polishing the photosensitive body in case of a photosensitive body made of relatively soft material such as OPC and by providing a mechanism for adjusting the photosensitive body to a predetermined temperature in case of a photosensitive body made of high hard material such as a-Si group. However, it is difficult to improve the service life of the apparatus and to make the apparatus compact, and detection and control of the latent image with high accuracy are required, with the result that the potential control requires a long time and is not efficient.
Further, in the above-mentioned method for macroscopically controlling the contrast, microscopically, the depth and the sharpness of the latent image may be varied in accordance with the environment or other factors. For example, as well as occurrence of the above-mentioned high humidity image flow, if the exposure amount is too great, since the latent image is expanded, the reproductively of the fine dots may not be ensured or the image may become dim (image flow).
In addition, in order to control the potential of the digital latent image which is changed in the width of several tens of .mu.m, the resolving power of the detecting means must be improved up to several tens of .mu.m. The resolving power can be improved to some extent by decreasing an area of the detecting portion of the potential sensor. However, as the area of the detecting portion is decreased, output of the detection signal is also reduced, thereby reducing S/N ratio.
On the other hand, in order to increase the output of the detection signal, a distance between the sensor and the drum must be approached up to several tens of .mu.m and a relative speed between the sensor and the drum must be increased. However, in the conventional sensors having the edge portion, discharge may be generated or measured potential distribution may be distorted. Further, when a discharge preventing means for preventing the discharge by using gas (SF.sub.6) having high discharge start voltage is provided, the apparatus itself becomes more complicated, and, if the relative speed between the sensor and the photosensitive drum is increased, leakage of gas will affect a bad influence upon the charging process.
Under such present circumstances wherein digitization has been progressed, in order to measure the change in the surface potential with the high resolving power of the order of several tens of .mu.m, it has been requested to provide a technique in which the resolving power is enhanced while maintaining the S/N ratio. The measuring method can preferably be effected simply and easily, and the control is effected on the basis of the measured result to obtain the high quality image stably. In particular, in the presently used compact electrophotographic apparatuses such as printers, it is effective to provide a system capable of extending service interval.